1. Field of the Invention
The present invention relates in general to a wheeled vehicle equipped with a roll rigidity control device and a slip control device for a differential, and more particularly to an integrated control system for a roll rigidity control device and a slip control device for a differential.
2. Description of the Prior Art
A known slip control device for a differential includes a hydraulically operated friction clutch unit which is variably engaged for varying a differential action as disclosed in Japanese Provisional Patent Publication Nos. 62-103226 and 62-103227.
In order to retain a sufficient supply of a driving force to the driving wheels during cornering of the vehicle, it is desirable for the friction clutch unit to be engaged more strongly, i.e., it is desirable to increase a slip limiting force (i.e., force supplied to the friction clutch unit to limit a slip thereof and therefore a differential action) during cornering of the vehicle.
When the slip limiting force is maintained high, it becomes possible to drive the vehicle, when cornering, in a power slide running state where the vehicle is subjected to a high lateral acceralation.
However, in a vehicle equipped with a so-called suspension which is capable of varying the roll rigidity in addition to the above described slip control device, the slip limiting force control by the slip control device and the roll rigidity control by the roll rigidity control device are performed independently and have no relation to each other. Due to this, at the initial stage of vehicle cornering in a power slide running state in which the slip limiting force is maintained high to prevent the differential movement of the driving wheels, a tendency of far strong understeer results.
With a view to reducing the tendency to understeer at the initial stage of such cornering, the assignee of this application has heretofore proposed a control device for controlling, at the initial stage of cornering, the distribution of the roll rigidity to the front and rear wheels in such a way that the roll rigidity is smaller on the front wheel side and larger on the rear wheel side as disclosed in the Japanese Patent Application 61-114269.
This control device is effective for reducing the tendency to understeer at the initial stage of cornering. However, as the magnitude of roll increases after the initial stage, the cornering inside wheel is lifted above the road surface, thus causing a reverse steering state of changing a vehicle steering characteristic from understeer to oversteer. Due to this, an excessive oversteer may result if the distribution of the roll rigidity is maintained in an oversteer effecting state, i.e., in such a state that the distribution of roll rigidity is smaller on the front wheel side and larger on the rear wheel side.